The invention relates to a magnetic-tape scanning device comprising a rotatable scanning means which is rotatable about a spindle axis and carries at least one magnetic head, and a rotary transformer; and more particularly to such a device which comprises a rotatable transformer ring, which is concentric with the spindle axis and which is secured to the rotatable scanning means, and a stationary transformer ring, which is secured to the stationary scanning means opposite the rotatable transformer ring so that a narrow axial air gap is formed between these rings. At least one of the two transformer rings is mounted via a plurality of cylindrical supports for the transformer ring, which supports have a cylindrical surface and first and second end faces, and quantities of glue fuming a first layer of glue between the first end face of each support and the transformer ring and a second layer of glue for securing the transformer ring to the relevant scanning means.
Such magnetic-tape scanning devices are employed in magnetic video-tape recorders, for example Philips type VR 2020. The transformer rings provide the contactless signal transfer from and to the rotating magnetic heads through the air gap between the two transformer rings. In this known magnetic-tape scanning device the stationary transformer ring is supported by three supports.
For an efficient signal transfer between the two transformer rings the axial spacing between the two transformer rings should be very small, of the order of magnitude of nominally 60 microns with a tolerance of approximately 30 microns to each side. This small dimension of the air gap should be maintained within a temperature range from -25.degree. C. to +75.degree. C. and for a relative humidity of 95% at 55.degree. C. Since the rotatable transformer ring is arranged on the rotatable scanning means, and the stationary transformer ring is on the stationary scanning means, the dimension of the air gap between the two transformer rings will depend on the accuracy of the respective mountings of the rings to the scanning means, and the length of the axial air gap between the rotatable and the stationary scanning means. This necessitates an accurate adjustment of the axial distance of each transformer ring relative to the air gap edge of the respective scanning means on which the transformer ring is mounted. When the rotatable scanning means is mounted on the stationary scanning means this axial air gap is adjusted accurately under standard conditions. Temperature variations of the magnetic-tape scanning device may affect the dimension of the axial air gap between the scanning means, which has a direct influence on the air gap between the two transformer rings.
In this known magnetic-tape scanning device the first end faces of the three supports are glued to that side of the transformer ring which is remote from the air gap by means of the first layer of glue. When the first layers of glue have cured the transformer ring is placed in a lathe and the second end faces of the supports are machined accurately. These second end faces serve to co-operate with a machined portion of the scanning means. The scanning means should also be machined accurately, in particular the axial distance between the air gap edge of the scanning means and this machined portion must be formed very accurately and with a very small tolerance of the order of magnitude of 10 microns. Now drops of glue are applied to that side of the transformer ring where the supports are situated, at locations between the supports. After this the transformer ring is placed in the scanning means, so that the applied drops of glue connect the transformer ring directly to the scanning means. Consequently, no layer of glue is present between the second end faces of the supports and the scanning means. This is because layers of glue at these locations would affect the axial position of the transformer ring and as a result of the sensitivity to moisture and heat of the known types of glue, would give rise to variations of said distance and thus to variations of the air gap between the two transformer rings.
It has been found that in the known magnetic-tape scanning device the thickness of the layer of glue between the transformer ring and the scanning means may still vary, in particular under the influence of atmospheric moisture, such that the layer of glue becomes permanently thicker, which results in an undesired reduction of the air gap between the transformer rings.